The present invention relates generally to the field of assembling heat exchange tubes and in particular to a new and useful non-metallic mandrel for insertion into a tube to provide hydraulic pressure and expand the tube against a surrounding tubesheet.
In the power plant field, a tubesheet is used for a nuclear steam generator, heat exchanger or a similar component that houses several thousand tube ends. The tube sheet has numerous pre-drilled holes which allows for each tube end to be inserted therethrough. The tube ends are welded to the tubesheet and circumferentially expanded into the tubesheet holes through virtually the full thickness of the tubesheet. This process is commonly referred to as full depth expansion.
The expansion of the tube ends can be achieved through mechanical or hydraulic processes. When manufacturing components for a nuclear steam generator, it is preferable that hydraulic expansion be used. Hydraulic expansion is the recommended method for nuclear steam generators because it produces less residual stress in the tube, and reduces the potential for stress corrosion cracking compared to other expansion methods. Hydraulically expanding the tube into a hole in the tubesheet closes the crevice between the tube and the hole thereby eliminating a potential corrosion site.
After being welding to the tubesheet, each tube is expanded into the tubesheet by inserting a hydraulic expansion mandrel into the tube. Steel hydraulic expansion mandrels are well known in the art, such as those manufactured by Haskel International, Inc. Other mandrels are described in U.S. Pat. No. 3,977,068, which illustrates a mandrel having a frusto-conical tip. The expansion zone of the mandrel, located between a pair of seals, such as O-rings, has a smaller diameter than the tip and the opposite end of the mandrel. A high pressure fluid, such as distilled water at 35,000 psi (2413 bar), is injected through the mandrel into the space between the smaller diameter portion of the mandrel and the tube wall to expand and seal the tube against the surrounding tubesheet. The mandrel can then be extracted from the expanded tube.
U.S. Pat. No. 4,802,273 teaches another mandrel having a particular seal configuration for isolating the reduced-diameter portion of the mandrel within a tube.
Neither of these two patents suggests using materials other than steel for the mandrel. Mandrels for expanding tubes known to the inventor are presently only made of steel.
Steel mandrels have been found to have some drawbacks. When the mandrel is moved in and out of the tubes, it is fairly common for the operators to inadvertently scratch, gall and mar the inside of the tubes via metal to metal contact between the tubes and the mandrel. Steel mandrels are difficult for people to operate, since the weight of steel significantly fatigues the operator after a period of use, e.g. after moving the mandrel in and out of the thousands of tubes that can be found in a large heat exchanger. Further still, steel mandrels are prone to problems due to mandrel stretch.
Mandrels having plastic sleeves are also known, but these mandrels have problems as well. In particular, if the sleeve should fail, operators may cause damage to tubes by thinking the mandrels are protected. Also, the plastic can pick up and become embedded with grit, which scratches and mars tubes when the xe2x80x9cprotectedxe2x80x9d mandrel is inserted and removed.
Further, on commercially available mandrels, the seals used to isolate the reduced-diameter section where the pressurized fluid is injected typically have many small parts. These seals tend to fail due to fatigue after long use, which causes additional damage to the tubes.
It is an object of the present invention to provide a tube expansion mandrel that does not scratch the tube to be expanded.
It is another object of the present invention to provide a tube expansion mandrel which is lightweight for easier use by operators.
It is a still further object of the invention to provide a tube expansion mandrel having few moving parts in order to reduce the likelihood of failure.
It is yet another object of the invention to provide a mandrel with improved tensile properties thereby reducing or eliminating problems due to mandrel stretch.
Accordingly, a hydraulic tube expansion mandrel is provided constructed of a fiber-reinforced material such as carbon fiber-reinforced material. O-ring seals are provided around a reduced-diameter section for isolating the region where a pressurized fluid is provided when the mandrel is inserted in a tube being expanded. An adjustment shim, located between a threaded collar and a locking stop collar, adjusts the length of the mandrel inserted into a tube.
Accordingly, a mandrel for hydraulically expanding a tube is provided which comprises an elongated cylinder having a tip, a reduced diameter section and a fluid supply end. A pair of O-rings separates the reduced diameter section from the tip and fluid supply end of the elongated cylinder. The elongated cylinder is made of a fiber-reinforced material
In an alternate embodiment, a mandrel for hydraulically expanding a tube is provided which comprises an elongated cylinder made of a carbon fiber-reinforced material produced via filament winding carbon fibers. The elongated cylinder has a tip, a reduced diameter section and a fluid supply end. A pair of O-rings separaties the reduced diameter section from the tip and fluid supply end of the elongated cylinder. A split threaded collar is secured to the fluid supply end and a locking stop collar having a front edge is fitted over the fluid supply end adjacent the reduced diameter section. An adjustment shim is located between the split threaded collar and the locking stop collar for adjusting the length to which the mandrel can be inserted into the tube.
In yet another embodiment, a method for hydraulically expanding a tube, comprises the following steps: a) providing an elongated cylinder made of a carbon fiber-reinforced material having a tip, a reduced diameter section and a fluid supply end, b) providing a pair of O-rings separating the reduced diameter section from the tip and fluid supply end of the elongated cylinder, c) inserting the elongated cylinder into a tube, and d) introducing a high pressure fluid via the fluid supply end into the reduced diameter section to hydraulically expand the tube.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which a preferred embodiment of the invention is illustrated.